In 1972, a belt driven tape cartridge design (U.S. Pat. No. 3,692,255, issued to Robert A. Von Behren) was introduced for use in the computer industry for digital tape recording. Cartridges commonly in use in 1972 were approximately 100 mm by 150 mm and the magnetic tape was 6.35 mm (0.250 inches) wide. Cartridges of that size are commonly called DC-6000 cartridges. DC-6000 cartridges and corresponding drive mechanisms are still commercially available. In 1976, smaller data cartridges were developed, as described by Alan J. Richards, Mini Data Cartridge: A Convincing Alternative for Low-Cost, Removeable Storage, Hewlett-Packard Journal, May, 1976. The mini-cartridge size is 63.5 mm by 82.5 mm. Mini-cartridges are commonly called DC-2000 cartridges. DC-2000 cartridges and corresponding drive mechanisms are still commercially available. Since both sizes of cartridges were first introduced, data densities, track densities, and tape speeds have all increased substantially. Further increases in track density are limited by the degree to which the tape can be prevented from wandering transversely within its intended path.
Both sizes of cartridges described above have two openings in one edgewall. One opening provides access to the tape by a magnetic head. The second opening provides access to an internal drive roller. The larger cartridges described above typically have three non-rotating tape guides along the edgewall containing the openings, so that there is a guide on each side of each opening with one guide in common. Therefore, when a magnetic head penetrates into the tape cartridge, the span of tape across the head is supported by two guides. Likewise, the span of tape across the drive roller is supported by two guides. In the larger size cartridges, each guide typically has a flange or shoulder on each end to provide a precise guidance path for the tape.
The three guides described above have been extensively analyzed for their effect on tape wear and guidance. For example, as disclosed in the Richards article, the surface finish of the guides is critical. If the guides are too rough, rapid tape wear results. If the guides are too smooth, the tape tends to adhere to the guides resulting in a high drive force and high-frequency tape speed variations. In addition, the three guides described above are the subject of several patents in which tape guidance is improved, primarily as a result of refinements to the guide flanges. U.S. Pat. No. 4,534,523 issued to Myron Zarr on Aug. 13, 1985 discloses guides where the flanges are arranged so that alternate guides provide a centering force against opposite edges of the tape. U.S. Pat. No. 5,104,058 issued to James A. Eggebeen on Apr. 14, 1992 discloses guides in which one guide has a wide clearance and the guides are arranged so that as the tape contacts a flange of the wider guide, the tape edges are forced against alternate flanges on the other two guides. Each of the patents just described addresses the effects of guide flanges on the edges of the tape but not the effects of the main guide surfaces in contact with the back surface of the tape or the effects of the length of the tape edge in contact with a flange.
In the larger size cartridges, the three guides typically all have the same diameter. Mini-cartridges may or may not have a central guide. Due to mechanical design constraints, in those mini-cartridges that have a central guide, the central guide is typically a small flangeless pin much smaller in diameter than the two outer guides. A larger diameter central guide would interfere with the internal drive roller. For example, U.S. Pat. No. 4,324,372 issued to Stepan Majicek and Harry R. Robinson on Apr. 13, 1982 discloses a mini-cartridge with three guides. In Majicek, two relatively large outer guides are provided with conically shaped flanges to help center the tape with reduced edge damage but the central guide is a simple relatively small diameter flangeless pin. The Zarr patent described above discloses a relatively small diameter central guide having one shoulder. U.S. Pat. No. 5,289,988 issued to David D. Madsen on Mar. 1, 1994 discloses a mini-cartridge having additional guides near the tape reels. The central guide in Madsen is a relatively small diameter flangeless pin. There is a need for further improvement in tape guidance in mini-cartridges.